Researchers also found an extremely hairy caterpillar which is now awaiting cataloguing in Oxford, where the team will give names to their finds.

It is estimated that along with the giant rat and cuscus the expedition found about 16 species of frogs, one species of gecko, at least three species of fish, at least 20 of insects and spiders and possibly one new species of bat.

The expedition was led by climber and naturalist Steve Backshall-wildlife cameraman Gordon Buchanan and head scientist Dr George McGavin.

Mr Buchanan and Smithsonian biologist Dr Kristofer Helgen were first on the scene when the rat was found by a tracker from the local Kasua tribe. Dr Helgen said:

'This is one of the world's largest rats. It is a true rat, related to the same kind you find in the city sewers, but a heck of a lot bigger.

'I had a cat and it was about the same size of this rat. This rat was incredibly tame.

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'It just sat next to me nibbling on a piece of leaf. It won't have seen a human being before. The crater of Mount Bosavi really is the lost world.'

Papua New Guinea is famous for the number and diversity of rodents that live there, with more than 57 species from the Murid family of rats and mice on the island.

Further evidence of the rich wildlife of the tropical location came with the discovery of the Bosavi Silky Cuscus. This animal, which resembles a small bear, is a marsupial that lives up in trees, feeding on fruits and leaves.

Weighing in at some 4.5lb, it has thick silky fur adapted for the mountain environment in which it lives. Dr Helgen has identified it as a new subspecies in the group of strange marsupials known as cuscuses.